مجله مکانیک سازه ها و شاره ها
سال چهاردهم شماره 5 (آذر و دی 1403)

In this study, numerical simulations are used to investigate the dynamic stall phenomenon at two advance ratios, μ = 0.3 and μ = 0.35, on a single blade with cyclic pitching motion. To simulate the three-dimensional compressible flow field, the unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved using the finite volume discretization method. A hybrid mesh is employed, and turbulence is modeled using the k-ω SST model. To validate and verify the numerical method, flight test data from the AH-1G helicopter was used. The comparison results confirm the accuracy and reliability of the numerical approach applied in this study. The findings of this study indicate that contrary to expectations, the intensity and number of stalls on the retreating side of the rotor at the studied radial section (r/R = 0.778) are greater at µ = 0.3 compared to µ= 0.35. On the advancing side, the stall is more severe at µ = 0.35, with the post-stall pitching moment coefficient differing by 89%, resulting in a significant increase in negative aerodynamic damping at this ratio. An analysis of the flow pattern reveals that the dynamic stall development at both advance ratios begins with flow separation near the trailing edge, which then propagates upstream while simultaneously bend to outboard region due to radial flow.

Cardiovascular diseases are the leading cause of death worldwide. Computer simulations of cardiac function are gradually becoming a powerful tool to better understand cardiac behavior and support clinical decision-making. However, right heart modeling is still in its early stages. Pulmonary hypertension is a pathophysiological disorder that may involve several clinical conditions. This study presents a fully coupled multiscale mathematical and numerical model of cardiac electromechanics with idealized biventricular geometry for normal subjects and pulmonary hypertension patients in COMSOL software. Muscle fibers that control electrical conduction and myocardial contraction, is one of the vital factors for the electromechanical simulation of the heart which was defined using a rule-based method. Windkessel blood circulation model was used to calculate systemic and pulmonary pressures. Blood circulation was modeled with a straightforward approach for the left and right ventricles. Lastly, alterations in the functional behavior of the heart were evaluated between individuals without pulmonary hypertension and healthy subjects. It was observed that right ventricular pressure increases in pulmonary arterial hypertension and ejection fraction decreases.

This research delves into the practical implications of the effect of exhaust gases on the wake region of an Audi car, which includes vortices and separated flowing from its surface. The findings could potentially inform future vehicle design and performance strategies. The car model's rear configuration is akin to the standard DrivAer Estateback car, which the Aerodynamics Institute of the Technical University of Munich has modeled. The SST k-ω turbulence model is selected for simulation using computational fluid dynamics, considering the problem's nature and the flow stability analysis. To validate the solution method, the numerical results obtained from flow simulation at a Reynolds number of 4.87×106 around the standard model are compared with the experimental results obtained from the wind tunnel. After validating the solution method, it was determined in the initial investigation that at the given Reynolds number, the drag coefficient of the Audi car with an exhaust system is 1.15 % lower than the drag coefficient of the car without an exhaust system. Furthermore, the effects of increasing the vehicle speed, the angle of the exhaust pipe outlet, and finally, the presence of an exhaust on one side of it on the value of the drag coefficient are examined.

Fatigue testing of composite structures is a destructive, costly and time-consuming process. In the search for a non-destructive procedure to predict the life of fiber reinforced polymer composites under cyclic loading, modal analysis has been proposed. Residual strength is considered as a popular indicator of damage accumulation and used in life prediction models. The relation between the residual strength and the modal parameters has been studied and reported. Results showed that with damage accumulation over fatigue cycling, a 20% decrease in residual strength (from 415 to 330MPa) corresponded to a 12% decrease in natural frequency (from 30.5 to 27Hz). Investigation of the fracture surfaces of fatigued specimens showed different failure micro-mechanisms for different maximum fatigue stress levels. The first mode of natural frequency not only followed the changes of residual strength with damage accumulation but also could reflect the structural changes and alteration of failure micro-mechanisms at different maximum stress levels. This paper focuses on how varying fatigue stress levels affect the failure micro-mechanisms and the relationship between these damages, residual strength and natural frequency. At lower maximum stress levels, more failure micro-mechanisms were activated and the changes in the first mode of natural frequency were more pronounced. This suggests that natural frequency is sensitive enough to detect different fatigue micro-mechanisms and could be used as an independent, non-destructive parameter for fatigue damage assessment.

This research investigates the quasi-3D buckling behavior of the functionally graded plate integrated with piezoelectric face sheets as a fundamental and sensitive sandwich structure in various industries is discussed based on the improved high-order shear theory by considering the stretching ‎effects along the thickness. The mechanical properties of the core are assumed to be ‎non-homogeneous‎ and the face sheets are modeled based on the theory of piezoelasticity. The equations of motion are extracted using the principle of ‎virtual work based on axial and biaxial loading conditions. After the ‎adaptation and evaluation of the present method, the critical buckling ‎loads of the three-layer plate ‎subjected to an external electric field are ‎calculated ‎in different conditions using Navier's analytical method. ‎Finally, the effects of various parameters including volume fraction ‎index, geometric dimensionless parameters, and comparison of two-‎dimensional and pseudo-three-dimensional models are examined. The ‎results show that ‎the difference between the 2D and quasi-3D models ‎‎decreases with the increase of the width-to-length ratio and the ‎decrease of the thickness. Eventually, the results show that the theory ‎used in modeling is very accurate in addition to being remarkably simple.‎

This research investigates the mechanical performance, ablation and oxidation of insulation based on carbon-epoxy Novalac 1179 reinforced with ZrB2/SiC ceramic particles with different grammage of fabric. The purpose of this research is to investigate the effect of carbon fabric grammage on epoxy novalac carbon insulation reinforced with ZrB2/SiC ceramic particles. Three samples including Novalac epoxy resin base with 60% SiC + 40% ZrB2 and 120, 200 and 600 gr/m2 of carbon fabric were made using hot press. The mechanical and thermal properties of insulation samples with different grammage using Oxyacetylene, bending and tensile tests were evaluated. To investigate the surface morphology, X-ray energy distribution elemental analysis (EDS) and SEM scanning electron microscope and X-ray diffraction (XRD) phase analysis have been used to identify the existing phases. The results showed that the mechanical properties and erosion resistance of the composite improved by increasing the grammage of the fabric. The biggest weight reduction and thickness reduction were in the insulation samples with 600 gr/m2 (G3) and 120 gr/m2 fabric(G1), respectively. Also, the G3 sample had the lowest temperature behind the insulation, below 150 degrees Celsius and had the highest mechanical properties, with 486.35 MPa of tensile strength and 677.18 MPa of bending strength. Finally, morphological studies showed that the presence of ZrB2/SiC particles leads to the creation of refractory phases such as ZrO2, SiO2 and ZrSIO4, which increase the resistance to hot erosion of carbon epoxy Novalac composites.

Due to the importance of the application of piezoelectric nanostructures and due to their mass and small size ‎at the nano level, parameters depending on the size and surface energy should be included in the theoretical ‎models of their dynamic analysis and mathematical modeling. In current work, some nonclassical controller ‎effects such as strain gradient (SGT), nonlocal (NLT) and Gurtin–Murdoch surface/interface (GMSIT) ‎theories are presented for analyzing of nonlinear vibration in piezoelectric nanoresonator (PENR) compared ‎to classical theory (CT). PENR subjected to nonlinear electrostatic excitation with direct (DC) and ‎alternating (AC) voltages and also visco-pasternak medium. For this analysis, Hamilton’s principle, ‎Galerkin technique, combination of Complex averaging method and arc-length continuation are used to ‎analyze nonlinear frequency response and stability analysis of PENR. The results show that ignoring small-‎scale and surface/interface effects give inaccurate predictions of vibrational response of the PENR. It is ‎indicated that in different boundary condition, material length scale and nonlocal scale parameters ‎respectively lead to decreasing and increasing of PENR stiffness and also the amplitude of oscillation and ‎the range of instability of non-classic theories of NLT and SGT are greater than that of the classical one. ‎Also changes of surface/interface parameters lead to decreasing or increasing the dimensionless natural ‎frequency, resonant frequency, resonance amplitude, nonlinear behavior and the system's instability of ‎PENR.‎

Laser shock peening is a mechanical surface treatment that is caused by laser beam radiation on the surface of the metal. This processing causes plastic deformation, and compressive residual stresses under the surface of metal. In addition, applying nanostructure coatings is one of the new methods to increase the resistance of components under corrosive environment and high temperature. The purpose of this article is to investigate laser shock peening on residual stress and fatigue corrosion of Inconel 792 in 75% Na_2 SO_4+15% NaCl+10% V_2 O_5 environment. For this purpose, a number of samples were subjected to laser shock peening and another group was coated with graphene oxide before laser processing. Then, all the samples were subjected to corrosive environment at high temperature, and then the fatigue test was performed. The result of this research showed that the laser shock peening operation increased the fatigue life of this alloy for 2.4 times.

Due to the drastic change in aerodynamic parameters, the problem of controlling the landing mode will be much more complicated than the high altitude flight. On the other hand, the presence of obstacles in the way of the bird's landing is another condition that should be considered in the design of the controller. Thus, according to the above, it is necessary to use the predictive controller to solve the problem. This controller inherently has a high resistance to model change. Also, if this controller is used in a restricted manner, it can be used to bypass obstacles in the path of movement during landing. The aim of this paper is to control the system for automatic landing by means of model-based pre-interlinear control. The reason for using the pre-interlinear controller is the presence of obstacles in the path of movement and the fulfillment of the constraints in the environment to remove the obstacles. As a final result, this controller is designed in such a way that the effect of external disturbances on the bird is minimized and the stability of the system is not jeopardized by the emergence of model uncertainties. Also, in this method, the effect caused by the delay of the external navigation system is taken into account in the closed loop system and the stability of the system is guaranteed. Finally, the proposed controller design is calculated for a real bird model and its performance simulation in the presence of obstacles, lateral and longitudinal wind is presented.

In this study, the vibrations of a folded plate made of auxetic cells were examined. Initially, the elastic constants and density of the auxetic plate were determined based on the geometrical and material parameters of unit cell. The folded plate was considered as two jointed rectangular plates. Utilizing the first-order shear deformation theory and applying Hamilton's principle, the equations of motion governing each plate and the boundary conditions at the plate's edges were derived. Next, employing the combined Levy-differential quadrature method, the transformed equations of motion from the partial type to ordinary one has been solved. Assembling the equations of motion with boundary and continuity relations leads to an eigenvalue problem that its solution can present the frequency response function of folded plate. To validate the results obtained from the Levy-differential quadrature solution, the auxetic plate was simulated by a finite element analysis software (ABAQUS), and the comparison results demonstrated the accuracy of presented analytical method. Finally, the effects of plate's geometrical parameters on the natural frequencies of the folded plate were investigated. The results showed that by changing the folding angle from 180 degrees to less, or in other words, by converting the flat plate to a folded plate, the natural frequency first increases and then remains almost constant.